1. Field of the Invention
The present invention relates to a power module, and particularly relates to a power module configured such that a power device chip is arranged within an outer casing and an electrode of the power device chip is connected to an external electrode that is integrated with the outer casing.
2. Description of the Background Art
In a conventional power module, an electrode of a power device chip arranged on an insulating substrate placed within a module casing is connected to an external electrode of the module casing by wire bonding using a aluminum wire, or the like (for example, see Japanese Patent Application Laid-Open No. 2-260551).
There is a power module having a high reliability, in which an electrode of a power device chip and an external electrode of a module casing are connected to each other by DLB (direct lead bonding) instead of the wire bonding, and the power device chip is sealed by using a transfer mold technology.
The wire bonding mentioned above is a proven method that has been performed since the 1980s. However, the aluminum wire is directly bonded to a heat generator of the power device chip. Therefore, heat stress caused by a repetition of turning-on and turning-off of the power device chip is applied to the aluminum wire, and this heat stress directly influences a fatigue life. Accordingly, when priority is given to an increased life span, designing with a maximum rated bonding portion temperature being set low is necessary, but such designing involves a problem of an increased size and increased costs of the power module.
The transfer mold technology mentioned above has an excellent reliability and mass production properties, but requires a large initial investment because a mold, or the like, should be prepared. Additionally, the bonding with the DLB restricts the degree of freedom in a terminal shape and the size of the external electrode. Thus, there is a problem that the costs are significantly increased when products having different design sizes are manufactured in a small lot.